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Producing a silica particle by inhibiting the generation of incompletely
reacted materials such as oligomers which have not been grown to the
silica particles having the intended particle size. A dispersion liquid
of a silica particle is produced by simultaneously adding, to a liquid
substantially consisting of an organic solvent, a liquid containing
silane alkoxide and a liquid containing an alkali catalyst and water to
cause hydrolysis and polycondensations to produce a silica particle. The
variation rate of the mole ratio of the alkali catalyst to silane
alkoxide in the reaction system for a period from the start to the end of
the reaction relative to the initial mole ratio is 0.90 to 1.10; and the
variation rate of the mole ratio of water to silane alkoxide for a period
from the start to the end of the reaction relative to the initial mole
ratio is 0.90 to 1.10.

1. A method for producing a dispersion liquid of a silica particle,
comprising: a step of providing a liquid I consisting substantially of an
organic solvent in a container; and a step of simultaneously adding a
liquid A containing silane alkoxide and a liquid B containing an alkali
catalyst and water to the liquid I to cause hydrolysis and
polycondensation of the silane alkoxide so as to produce a silica
particle.

2. The method for producing a dispersion liquid of a silica particle
according to claim 1, wherein a variation rate of a mole ratio of the
alkali catalyst to silane alkoxide in a reaction system during a period
from a start to an end of an addition of the liquids A and B relative to
an initial mole ratio of the alkali catalyst to silane alkoxide is 0.90
to 1.10; and the variation rate of the mole ratio of water to silane
alkoxide in the reaction system during the period from the start to the
end of the addition of the liquids A and B relative to the initial mole
ratio of water to silane alkoxide is 0.90 to 1.10.

3. The method for producing a dispersion liquid of a silica particle
according to claim 1, wherein the mole ratio of the alkali catalyst to
silane alkoxide in the reaction system during a period from the start to
the end of the addition of the liquids A and B is constantly 0.20 or
more; and the mole ratio of water to silane alkoxide in the reaction
system during the period from the start to the end of the addition of the
liquids A and B is constantly 2.0 or more.

4. The method for producing a dispersion liquid of a silica particle
according to claim 1, wherein a pH in the reaction system at the end of
the addition of the liquids A and B is 11 or higher.

5. A dispersion liquid of a silica particle comprising a silica particle
having an average particle diameter (d) of 5 to 300 nm, wherein a content
of incompletely reacted material is 200 ppm or less.

6. The dispersion liquid of a silica particle according to claim 5,
wherein the silica particle includes a content of each of U and Th of
less than 0.3 ppb, a content of each of alkali metals, alkali earth
metals, Fe, Ti, Zn, Pd, Ag, Mn, Co, Mo, Sn, Al, and Zr of less than 0.1
ppm, and a content of each of Cu, Ni, and Cr of less than 1 ppb.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The present invention relates to a method for producing a
dispersion liquid of a silica particle, and particularly to a method for
producing a dispersion liquid of a silica particle useful for an abrasive
used in polishing during formation of a metal wiring layer on a
semiconductor integrated circuit, etc.

Description of the Related Art

[0002] Various integrated circuits are used for computers and various
electronic devices. As they are downsized and become sophisticated,
higher density and higher performance have been required for circuits.

[0003] To produce a semiconductor integrated circuit, for example, an
interlayer film (insulating film) is formed on a substrate such as a
silicon wafer, a groove pattern for a metal wiring is formed on the
interlayer film (insulating film), a barrier metal layer of tantalum
nitride (TaN) or the like is formed by a sputtering method or the like,
as necessary, and then a copper film for a metal wiring is formed by a
chemical vapor deposition (CVD) method or the like. Here, when a barrier
metal layer of TaN or the like is provided, the barrier metal layer can
prevent, for example, lowering of the insulation properties of the
interlayer insulating film in association with the diffusion of copper or
an impurity or the like to the interlayer insulating film and erosion of
the interlayer insulating film, and can enhance the adhesion between the
interlayer insulating film and copper.

[0004] Subsequently, a film of unnecessary copper and barrier metal
(occasionally referred to as "sacrifice layer") formed on a portion out
of the groove is removed by polishing with a chemical-mechanical
polishing (CMP) method and the upper surface is flattened to the maximum
extent possible, and thus a wiring/circuit pattern of copper is formed by
leaving a metal film only in the groove.

[0005] The abrasive used in the CMP method is typically produced by adding
an oxidant for increasing the polishing rate of a wiring/circuit metal
and an additive such as organic acid to a dispersion liquid comprising a
spherical polishing particle consisting of a metal oxide such as silica
and having an average particle diameter of about 5 to 300 nm.

[0006] When incompletely reacted materials (by-products) such as oligomers
of silane alkoxide are present in the dispersion liquid comprising the
polishing particle (silica sol), the silica sol has not had a sufficient
stability, presumably due to the influence of the incompletely reacted
materials such as highly reactive oligomers. Further, thickening,
aggregation, white turbidity, generation of a sedimentary gel, etc.
sometimes have occurred due to the influence of additives mixed to the
dispersion liquid when used as an abrasive. Use of such an abrasive
sometimes has resulted in appearance of scratches due to aggregates and
has caused a problem by silica components remaining on a substrate after
polishing (e.g., see Japanese Patent Laid-Open Publication Nos.
2015-124231, 2012-156393, and 2014-154707). Furthermore, in some cases,
the silica sol has adsorbed an additive for enhancing the abrasive
property, resulting in reduction in the effect of the additive.

[0007] As a method for producing a silica sol in which the generation of
incompletely reacted materials such as such oligomers is suppressed, for
example, proposed is a method which comprises (a) a step of adding an
organic solvent containing tetramethoxysilane and a solvent containing an
alkali catalyst and water to an organic solvent containing an alkali
catalyst and water to cause hydrolysis and polycondensation of
tetramethoxysilane, so as to produce a silica sol; and (b) a step of
heating a dispersion medium of the silica sol to a boiling point of water
to substitute with water (see Japanese Patent No. 4566645).

[0012] The method described in Japanese Patent Laid-Open No. 2015-124231
produces a highly-pure silica particle in a good productivity but, in
step (a), generates incompletely reacted materials such as oligomers of
silane alkoxide which do not grow to the silica particles which are
intended to be produced thus requires step (b) for removing the
incompletely reacted materials. Therefore, this method has a problem from
the standpoint of production efficiency, cost, etc.

[0013] The present invention has been made in view of the above
circumstances. An object of the present invention is to provide a method
for producing a dispersion liquid of a silica particle in which the
silica particle is efficiently produced, by suppressing the generation of
incompletely reacted materials such as oligomers which do not grow to
silica particles which are intended to be produced.

SUMMARY OF THE INVENTION

[0014] As a result of diligent research to solve the above-described
problems, the present inventors have found that the generation of
incompletely reacted materials such as oligomers which do not grow to
silica particles which are intended to be produced is suppressed by
simultaneously adding a liquid A containing silane alkoxide and a liquid
B containing an alkali catalyst and water to a liquid I consisting
substantially of an organic solvent in a container, and have completed
the present invention. Particularly, they have found that by keeping the
amounts of water and alkali catalyst which have a great influence on the
hydrolysis of silane alkoxide to be constant relative to the amount of
the silane alkoxide during the reaction period, the silane alkoxide which
is sequentially added can always hydrolyze under the same conditions, and
the generation of incompletely reacted materials such as oligomers which
do not grow to silica particles which are intended to be produced is thus
suppressed.

[0015] Accordingly, the present invention relates to a method for
producing a dispersion liquid of a silica particle, comprising a step of
providing a liquid I consisting substantially of an organic solvent in a
reaction container; and a step of simultaneously adding a liquid A
containing silane alkoxide and a liquid B containing an alkali catalyst
and water to the liquid I, to cause hydrolysis and polycondensation of
the silane alkoxide, so as to produce a silica particle.

[0016] The method is preferably a method for producing a dispersion liquid
of a silica particle that satisfies the following provisions (1) and (2):

[0017] (1) the variation rate of the mole ratio of the alkali catalyst to
silane alkoxide (alkali catalyst/silane alkoxide) in the reaction system
during a period from a start to an end of an addition of the liquids A
and B (a period from the start of the reaction (start of the addition) to
the end of the reaction) relative to the initial mole ratio of the alkali
catalyst to silane alkoxide is 0.90 to 1.10; and

[0018] (2) the variation rate of the mole ratio of water to silane
alkoxide (water/silane alkoxide) in the reaction system during the period
from the start to the end of the addition of the liquids A and B (a
period from the start of the reaction (the addition) to the end of the
reaction) relative to the initial mole ratio of water to silane alkoxide
is 0.90 to 1.10.

[0019] Hereinafter in the present specification, the expression "variation
rate of the mole ratio of the alkali catalyst to silane alkoxide in the
reaction system during a period from the start to the end of the addition
of the liquids A and B relative to the initial mole ratio" is simply
referred to as "variation rate of catalyst proportion," and the
expression "variation rate of the mole ratio of water to silane alkoxide
in the reaction system during a period from the start to the end of the
addition of the liquids A and B relative to the initial mole ratio" is
simply referred to as "variation rate of water proportion." The
expression "period from the start to the end of the addition of the
liquids A and B" is referred to as "period from the start to the end of
the reaction."

[0020] It is preferred that, in the production method according to the
present invention, the mole ratio of the alkali catalyst to silane
alkoxide in the reaction system during a period from the start to the end
of the reaction is constantly 0.20 or more, and the mole ratio of water
to silane alkoxide in the reaction system during a period from the start
to the end of the reaction is constantly 2.0 or more. Further, the pH in
the reaction system at the end of the reaction is preferably 11 or
higher.

[0021] The present invention also relates to a dispersion liquid of a
silica particle comprising a silica particle having an average particle
diameter (d) of 5 to 300 nm, wherein a content of incompletely reacted
materials in the dispersion liquid is 200 ppm or less.

[0022] It is preferred that, in the dispersion liquid of a silica particle
according to the present invention, the silica particle includes a
content of each of U and Th of less than 0.3 ppb, a content of each of
alkali metals, alkali earth metals, Fe, Ti, Zn, Pd, Ag, Mn, Co, Mo, Sn,
Al, and Zr of less than 0.1 ppm, and a content of each of Cu, Ni, and Cr
of less than 1 ppb.

[0023] According to the method for producing a dispersion liquid of a
silica particle of the present invention, the silica particle can be
efficiently produced by suppressing the generation of incompletely
reacted materials such as oligomers which do not grow to silica particles
which are intended to be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 shows diagrams illustrating a method for calculating the
average particle diameter (d) in the present invention, wherein each
solid black area represents an interparticle junction part, and each
interparticle junction part may include a space.

[0025] FIG. 2 shows diagrams illustrating a method for calculating the
aspect ratio (b/a; provided that b a) in the present invention, wherein a
represents a short axis diameter and b represents a long axis diameter,
each solid black area represents an interparticle junction part, and each
interparticle junction part may include a space.

[0026] FIG. 3 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 1.

[0027] FIG. 4 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 2.

[0028] FIG. 5 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 3.

[0029] FIG. 6 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 4.

[0030] FIG. 7 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 5.

[0031] FIG. 8 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Example 6.

[0032] FIG. 9 is a graph showing the chronological change in the variation
rate of catalyst proportion and the variation rate of water proportion in
Comparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Method for Producing Dispersion Liquid of Silica Particle]

[0033] The method for producing a dispersion liquid of a silica particle
according to the present invention is a method for producing a silica
particle by simultaneously adding a liquid A containing silane alkoxide
and a liquid B containing an alkali catalyst and water to a liquid I
consisting substantially of an organic solvent to cause hydrolysis and
polycondensation of the silane alkoxide. It is preferred that, in the
method for producing a dispersion liquid of a silica particle according
to the present invention, the variation rate of the mole ratio of the
alkali catalyst to silane alkoxide in the reaction system during a period
from the start to the end of the reaction relative to the initial mole
ratio is 0.90 to 1.10; and the variation rate of the mole ratio of water
to silane alkoxide in the reaction system during a period from the start
to the end of the reaction relative to the initial mole ratio is 0.90 to
1.10.

[0034] In the method for producing a dispersion liquid of a silica
particle, a liquid A containing silane alkoxide and a liquid B containing
an alkali catalyst and water are simultaneously added to a liquid I
consisting substantially of an organic solvent to keep constant the
amounts of water and an alkali catalyst relative to the amount of the
silane alkoxide during the reaction period from the start to the end of
the reaction, so that the silane alkoxide which is sequentially added is
always hydrolyzed under the same conditions. As such, the generation of
incompletely reacted materials such as oligomers which do not grow to
silica particles which are intended to be produced is thus suppressed.
This makes it possible to eliminate a step of removing incompletely
reacted materials and thus to efficiently produce a dispersion liquid of
a silica particle. As the produced dispersion liquid of a silica particle
does not contain incompletely reacted materials such as oligomers, it can
thus provide an abrasive having an excellent stability as a dispersion
liquid of a silica particle and an abrasive, and having a good abrasive
property.

<Liquid I (Liquid Provided in Advance in Container)>

[0035] A liquid I consists substantially of an organic solvent. Examples
of the organic solvent include an alcohol, a ketone, an ether, a glycol,
and an ester, with an alcohol being preferred. More particularly,
alcohols such as methanol, ethanol, propanol, and butanol; ketones such
as methyl ethyl ketone and methyl isobutyl ketone; glycol ethers such as
methyl cellosolve, ethyl cellosolve, and propylene glycol monopropyl
ether; glycols such as ethylene glycol, propylene glycol, and hexylene
glycol; and esters such as methyl acetate, ethyl acetate, methyl lactate,
and ethyl lactate can be exemplified. Among them, methanol or ethanol is
more preferred, and methanol is particularly preferred. These organic
solvents may be used alone or in a mixture of two or more.

[0036] The expression "consist(ing) substantially of an organic solvent"
used herein means that the inevitably contained impurities or the like
derived from the process of producing the organic solvent can be
contained but the other impurities or the like are not contained. For
example, the content of the organic solvent is 99% by mass or more, and
preferably 99.5% by mass or more.

[0037] In a conventional reaction system in which an alkali catalyst and
water are placed in a liquid I, the composition in the system changes
sequentially from the start of the reaction, and the hydrolysis
conditions of the silane alkoxide are not constant. Further, the pH in
the system is high at the start of the reaction but tends to decrease
thereafter, and when the amount of alkali catalyst to be added is
insufficient, the pH often becomes below 11 at the end of the reaction
and incompletely reacted materials tend to generate. However, according
to the present invention, a liquid I consisting substantially of an
organic solvent is used and the generation of the incompletely reacted
materials can thus be inhibited.

<Liquid A>

[0038] The liquid A contains silane alkoxide, and preferably further
contains an organic solvent. Typically, it is substantially consisted of
silane alkoxide, or substantially consisted of two components, i.e.
silane alkoxide and an organic solvent. In a similar manner as described
above, the expressions "substantially consist(ing) of silane alkoxide"
and "substantially consist(ing) of two components" used herein mean that
the inevitably contained impurities or the like derived from the process
of generating the silane alkoxide or the organic solvent can be contained
but other impurities or the like are not contained, and the content of
the organic solvent or the content of silane alkoxide and the organic
solvent is 99% by mass or more, and preferably 99.5% by mass or more.

[0039] Examples of the silane alkoxide include silane alkoxides
represented by the following formula [1].

X.sub.nSi(OR).sub.4-n [1]

[0040] In the formula, X represents a hydrogen atom, a fluorine atom, an
alkyl group having 1 to 8 carbon atoms, an aryl group, or a vinyl group;
R represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms,
an aryl group, or a vinyl group; and n is an integer of 0 to 3.

[0042] Among these, silane alkoxides represented by the formula [1]
wherein n is 0 and the alkyl chain of R is short, such as
tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), are particularly
preferably used. This is because when these are used, the hydrolysis rate
tends to be high and the incompletely reacted materials tend not to
remain. Among them, tetramethoxysilane (TMOS) having a short alkyl chain
is preferred.

[0043] The organic solvent to be used can be the organic solvents
exemplified regarding the liquid I, but the organic solvent having the
same composition as the liquid I is preferably used. That is, when
methanol is used in the liquid I, methanol is also preferably used in the
liquid A.

[0044] When the liquid A contains an organic solvent, the concentration of
silane alkoxide relative to the organic solvent is, for example, 1.5 to
6.4 mol/L, and preferably 2.0 to 6.0 mol/L.

<Liquid B>

[0045] The liquid B contains an alkali catalyst and water, and usually
consists substantially of two components. The expression "consist(ing)
substantially of two components" used herein have the same meaning as
described above regarding the liquid A.

[0046] Examples of the alkali catalyst to be used include basic compounds,
such as ammonia, an amine, an alkali metal hydride, an alkali earth metal
hydride, alkali metal hydroxide, an alkali earth metal hydroxide, a
quaternary ammonium compound, and an amine-based coupling agent, with
ammonia being preferred.

[0047] Here, the concentration of the alkali catalyst relative to water
is, for example, 1 to 24 mol/L, and preferably 3 to 15 mol/L.

<Reaction Conditions, Etc.>

[0048] As described above, the method for producing a dispersion liquid of
a silica particle according to the present invention is a method for
producing a silica particle by simultaneously adding a liquid A
containing silane alkoxide and a liquid B containing an alkali catalyst
and water to a liquid I consisting substantially of an organic solvent to
cause hydrolysis and polycondensation of the silane alkoxide, and
preferably satisfies the following two provisions:

[0049] (1) the variation rate of the mole ratio of the alkali catalyst to
silane alkoxide in the reaction system relative to the initial mole ratio
during a period from the start to the end of the reaction (variation rate
of catalyst proportion) is 0.90 to 1.10; and

[0050] (2) the variation rate of the mole ratio of water to silane
alkoxide in the reaction system relative to the initial mole ratio during
a period from the start to the end of the reaction (variation rate of
water proportion) is 0.90 to 1.10.

[0051] The production method according to the present invention is a
method in which the variation rate of catalyst proportion and the
variation rate of water proportion during a period from the start to the
end of the reaction are intended to be reduced as much as possible and to
keep the variation rates constant. The specific embodiments of the
production method according to the present invention include a method for
suppressing the variation rate of catalyst proportion and the variation
rate of water proportion by keeping the amounts of the alkali catalyst
and water contained in the liquid I as low as possible. The embodiments
also include a method for suppressing the variation rate of catalyst
proportion and the variation rate of water proportion by keeping the
addition conditions such as addition rates of the liquid A and the liquid
B as constant as possible during a period from the start to the end of
the reaction. For example, the changes in the addition rates of the
liquid A and the liquid B can be suppressed by using a highly accurate
pump.

[0052] Each of "the mole ratio of an alkali catalyst to silane alkoxide
(alkali catalyst/silane alkoxide)" and "the mole ratio of water to silane
alkoxide (water/silane alkoxide)" as used herein refers to the mole ratio
calculated based on the measured addition weights, assuming that the
hydrolysis and polycondensation reactions of silane alkoxide take place
instantaneously and the alkali catalyst is not released out of the
system. Each of the variation rate of catalyst proportion and the
variation rate of water proportion is determined at the predetermined
time intervals (for example, every 10 minutes) from the numerical value
obtained by calculating the mole ratio from the measured addition weights
and dividing the calculated value by the initial mole ratio. The initial
value (initial mole ratio) refers to the mole ratio (theoretical value)
immediately after addition of the liquid A and the liquid B.

[0053] In the method for producing a dispersion liquid of a silica
particle, the variation rate of catalyst proportion is preferably 0.90 to
1.10 as described above, more preferably 0.95 to 1.05, and still more
preferably 0.98 to 1.02.

[0054] Also, in the method for producing a dispersion liquid of a silica
particle, the variation rate of water proportion is preferably 0.90 to
1.10 as described above, more preferably 0.95 to 1.05, and still more
preferably 0.98 to 1.02.

[0055] Further, it is preferred that, in the present invention, the mole
ratio of the alkali catalyst to silane alkoxide in the reaction system
during a period from the start to the end of the reaction is constantly
0.20 or more and the mole ratio of water to silane alkoxide in the
reaction system for a period from the start to the end of the reaction is
constantly 2.0 or more. That is, it is preferred that, during the
reaction, each of the amounts of the alkali catalyst and water relative
to the amount of silane alkoxide is kept at a predetermined value or
more. By subjecting to the reaction the alkali catalyst and water the
amounts of which are kept at the predetermined value or more relative to
the amount of silane alkoxide, it is possible to allow hydrolysis to
sufficiently proceed, and inhibit an unreacted silane alkoxide from
remaining and incompletely reacted materials from generating.

[0056] Each of the mole ratio of the alkali catalyst to silane alkoxide
and the mole ratio of water to silane alkoxide refers to the mole ratio
calculated based on the measured addition weights as described above.

[0057] As described above, the mole ratio of the alkali catalyst to silane
alkoxide in the reaction system during a period from the start to the end
of the reaction is preferably constantly 0.20 or more, more preferably
constantly 0.30 or more, and still more preferably constantly 0.50 to
1.00.

[0058] Also, as described above, the mole ratio of water to silane
alkoxide in the reaction system during a period from the start to the end
of the reaction is preferably constantly 2.0 or more, more preferably
constantly 3.0 or more, and still more preferably constantly 3.5 to 15.0.

[0059] Further, the pH in the reaction system at the end of the reaction
is preferably 11 or higher, and more preferably 11.2 or more. In a
conventional reaction system in which an alkali catalyst is placed in a
liquid I, the pH in the system is often below 11 at the end of the
reaction, which is a cause of the generation of the incompletely reacted
materials. However, in the production method according to the present
invention, as described above, the pH at the end of the reaction can be
11 or higher by adding the alkali catalyst and water the amounts of which
are kept constant relative to the amount of silane alkoxide.

[0060] In the production method according to the present invention, the
reaction is usually performed under atmospheric pressure at a temperature
equal to or lower than the boiling point of the solvent to be used, and
preferably at a temperature lower by about 5 to 10.degree. C. than the
boiling point. After the reaction, water substitution (exchange of the
solvent for water) is performed as necessary.

[0061] For a dispersion liquid of a silica particle produced by the
production method according to the present invention, the generation of
the incompletely reacted materials such as oligomers of silane alkoxide
is suppressed, and therefore, it is not always necessary to perform
heat-ripening treatment, heat removal treatment, and purification
treatment such as ultrafiltration, which have been conventionally
performed.

[Dispersion Liquid of Silica Particle]

[0062] The dispersion liquid of a silica particle according to the present
invention comprises a silica particle having an average particle diameter
(d) of 5 to 300 nm, wherein the dispersion liquid includes a content of
incompletely reacted materials of 200 ppm or less. The dispersion liquid
of a silica particle can be produced by the above-described production
method according to the present invention. The dispersion liquid of a
silica particle is useful as an abrasive, and may be used as it is or in
a dried state.

[0063] The incompletely reacted materials refer to silicon-containing
compounds for which the reaction has not proceeded to provide the silica
particles intended to be produced, for example, unreacted raw material
silane alkoxide and its hydrolyzates (oligomers) having a low molecular
weight, specifically silicon-containing compounds present in a
supernatant obtained when centrifuging an aqueous dispersion liquid of a
silica particle at 10.degree. C., 1,370,000 rpm (1,000,000 G) for 30
minutes with CS150GXL, a micro ultracentrifuge manufactured by Hitachi
Koki Co., Ltd.

(Measurement Method of Content of Incompletely Reacted Materials)

[0064] The silicon-containing compounds (incompletely reacted materials)
present in the above supernatant are subjected to ICP emission
spectrometer ICPS-8100 manufactured by Shimadzu Corporation to measure
the Si contents, and the comparison is performed with the SiO.sub.2
concentrations calculated from the Si contents.

[0065] Since the dispersion liquid of a silica particle does not contain
the incompletely reacted materials such as oligomers, when it is used as
or in an abrasive, the deposits can be suppressed from generating on
substrates, and the adsorption of and reaction with various chemicals
added to the abrasives can be suppressed to exert effects of the
chemicals more effectively.

[0066] The silica particles contained in the dispersion liquid of a silica
particle form a three-dimensional polycondensation structure. This is due
to the fact that hydrolysis and polycondensation of silane alkoxide
occurring in an alkaline environment proceed not only in a planar state
(in a two-dimensional manner) but also in a cubic manner (in a
three-dimensional manner). An abrasive using a particle having such a
structure is suitable because it has a high particle dispersibility and
provides a sufficient polishing rate. On the other hand, hydrolysis and
polycondensation of the silane alkoxide occurring in an acidic
environment proceed in a two-dimensional manner, and hence a spherical
particle cannot be obtained.

[0067] The structure can be determined by the presence of a particle
observed with a transmission electron microscope or a scanning electron
microscope.

[0068] The average particle diameter (d) of a silica particle contained in
the dispersion liquid of a silica particle is 5 to 300 nm, and can be
appropriately set in accordance with a required polishing rate, polishing
precision, and so on. The method for calculating an average particle
diameter (d) is described with reference to FIG. 1. FIG. 1 illustrates a
particle in which a primary particle is present alone and a particle in
which a plurality of primary particles are linked to each other, wherein
each solid black area represents an interparticle junction part and each
interparticle junction part may include a space. The particle diameter d
is the longest diameter measured for the primary particle contained in
each particle. The average particle diameter (d) is determined as
follows: an electron micrograph of a silica particle is taken and 100
particles are arbitrarily selected therefrom; the longest diameter d of
the primary particle is measured for each of the 100 particles; and the
average value is used as the average particle diameter (d).

[0069] When the average particle diameter is smaller than 5 nm, the
dispersion liquid of a silica particle tends to have insufficient
stability, and the particle diameter is too small to achieve a sufficient
polishing rate. When the average particle diameter is larger than 300 nm,
scratches are likely to generate and insufficient smoothness is not
obtained in some cases when the silica particle is used as an abrasive,
although depending on the types of substrate or insulating film. The
average particle diameter is preferably 10 to 200 nm, and more preferably
15 to 100 nm.

[0070] The silica particle contained in the dispersion liquid of a silica
particle may be a true spherical-shaped particle having an aspect ratio
of 1.00 to 1.20, but it is preferably an irregular-shaped particle having
an aspect ratio of more than 1.20 and 5.00 or less. The irregular-shaped
particle having an aspect ratio within the above range has a convex
portion in its surface. As a result, stress is concentrated to the convex
portion in polishing, resulting in a higher polishing rate in polishing
the substrate.

[0071] The method for calculating an aspect ratio is described with
reference to FIG. 2. FIG. 2 illustrates a particle in which a primary
particle is present alone and a particle in which a plurality of primary
particles are linked to each other, wherein each solid black area
represents an interparticle junction part and each interparticle junction
part may include a space. The aspect ratio is determined as follows:
particles are observed under a scanning electron microscope; the long
side of a rectangle enclosing a particle is defined as the side b and the
short side is defined as the side a as illustrated in FIG. 2 and the
vertical-to-horizontal ratio (b/a) is measured for each of 100 particles;
and the average value is used as the aspect ratio.

[0072] It is preferred that, in the dispersion liquid of a silica
particle, the silica particle includes a content of each of U and Th of
less than 0.3 ppb, a content of each of alkali metals, alkali earth
metals, Fe, Ti, Zn, Pd, Ag, Mn, Co, Mo, Sn, Al, and Zr of less than 0.1
ppm, and a content of each of Cu, Ni, and Cr of less than 1 ppb. By being
within this range, the silica-based particle can be used as an abrasive
grain for highly integrated logics and memories with a wiring node of 40
nm or less, and for three-dimensional implementation.

[0073] When the metal elements as impurity components are present in
quantities larger than the above-mentioned ranges, the metal elements may
remain on a substrate polished with the silica particle, and cause
insulation failure to a circuit formed on a semiconductor substrate or
short the circuit to decrease the dielectric constant of a film for
insulation (insulating film) and increase the impedance of the metal
wiring, leading to lowering of the response speed, increase of the power
consumption, and so on. In addition, the metal element ions may migrate
(diffuse), and the failures may be caused under some conditions for use
or after a long-term use. In particular, U and Th generate radiation to
cause malfunctions to a semiconductor device even when the amount of
remaining U or Th is minute. Thus, the U or Th content higher than the
above range is not preferred.

[0075] To obtain such a highly-pure silica particle with a small content
of impurity components, it is preferred to use an apparatus the material
of which is free of such elements and has high chemical resistance in
preparation of the particle. Specific preferred examples of the material
include plastics such as Teflon.RTM., FRP, and carbon fibers, and
non-alkali glass.

[0076] In addition, it is preferred to purify raw materials to be used by
distillation, ion exchange, or removal with a filter. In particular,
alcohol used in hydrolysis of alkoxide, may be contaminated with metal
impurity components from a tank and so on or with a catalyst during
synthesis, and may require purification at a particularly high level.

[0077] As a method to obtain a highly-pure silica particle, it is possible
to provide raw materials with a small content of impurity components in
advance, or preventing contamination from an apparatus for particle
preparation, as described above. As another method, it is possible to
reduce impurity components for a particle prepared without taking such
countermeasures sufficiently. However, as impurity components are
incorporated in the silica particle, purification using ion exchange or
removal with a filter would be inefficient, and high cost may be
required. Thus, it is not practical for obtaining a silica particle with
a small content of impurity components with such a method.

[0078] To determine the content of U and Th, the contents of alkali
metals, alkali earth metals, Fe, Ti, Zn, Pd, Ag, Mn, Co, Mo, Sn, Al, and
Zr, and the contents of Cu, Ni, and Cr in the silica particle, the silica
particle is dissolved in hydrofluoric acid, and heated to remove the
hydrofluoric acid and then added with pure water as necessary, and the
resulting solution is subjected to measurement with an inductively
coupled plasma (ICP) emission mass spectrometer (for example, ICPM-8500
manufactured by Shimadzu Corporation).

EXAMPLES

[0079] Hereinafter, the present invention will be described with reference
to Examples. However, the present invention shall be not limited to these
Examples.

Example 1

<Production of Dispersion Liquid of Silica Particle (SA)>

[0080] 410.0 g of methanol (liquid I) was kept at 40.degree. C., and
3436.3 g of a methanol solution of tetramethoxysilane (manufactured by
Tama Chemicals Co., Ltd. (the same applies hereinafter) (liquid A) and
1684.0 g of aqueous ammonia (liquid B) were simultaneously added to the
liquid I over 10 hours. After completion of the addition, the mixture was
further aged at the same temperature for 3 hours. The solvent was
substituted with pure water to obtain 20% by mass of a dispersion liquid
of a silica particle (SA). Detailed processing conditions and results of
various measurements are shown in Table 1. The chronological changes with
time in the variation rate of catalyst proportion and the variation rate
of water proportion are shown in FIG. 3.

<<Mole Ratios of Alkali Catalyst and Water to Silane Alkoxide, and
Variation Rates of the Mole Ratio>>

[0081] Each of the mole ratios of alkali catalyst/silane alkoxide and
water/silane alkoxide was calculated based on the measured addition
weights, assuming that the hydrolysis and polycondensation reactions of
silane alkoxide take place instantaneously and the alkali catalyst is not
released out of the system. Each of mole ratios in the reaction system
was calculated from 10 minutes after the start of the addition of the
liquids A and B and every 10 minutes thereafter. The variations of the
mole ratios of the substances in the reaction system were compared with
each other by using the numerical values obtained by dividing the mole
ratios by the mole ratios (theoretical values) immediately after the
addition of the liquid A and the liquid B as the initial values.

Si(OR).sub.4+H.sub.2O.fwdarw.Si(OH).sub.4+4ROH

(4 mol consumed during hydrolysis)

Si(OH).sub.4.fwdarw.SiO.sub.2+2H.sub.2O

(2 mol released during polycondensation)

<<Measurement of Average Particle Diameter>>

[0082] The average particle diameter was determined as follows: an
electron micrograph of a silica particle was taken and 100 particles were
arbitrarily selected therefrom; the longest diameter of the primary
particle was measured for each of the 100 particles as illustrated in
FIG. 1; and the average value was used as the average particle diameter.

<<Measurement of Aspect Ratio>>

[0083] The aspect ratio was determined as follows: an electron micrograph
of a silica particle was taken; the long side of a rectangle enclosing a
particle was defined as the side (b) and the short side as the side (a)
as illustrated in FIG. 2 and the vertical-to-horizontal ratio (b/a) was
measured for each of 100 particles arbitrarily selected; and the average
value was used as the aspect ratio.

<<Measurement of Amounts of Incompletely Reacted Materials>>

[0084] For the amounts of incompletely reacted materials,
silicon-containing compounds (incompletely reacted materials) present in
a supernatant obtained when centrifuging the above-obtained 20% by mass
of dispersion liquid of a silica particle at 10.degree. C., 1,370,000 rpm
(1,000,000 G) for 30 minutes with CS150GXL micro ultracentrifuge
manufactured by Hitachi Koki Co., Ltd. were subjected to ICP emission
spectrometer ICPS-8100 manufactured by Shimadzu Corporation to measure
the Si contents, and the comparison was performed with the SiO.sub.2
concentrations calculated from the Si contents.

<<Measurement of Concentration of Silica Particle in Reaction
System>>

[0085] 5 g of each of samples was dried at 150.degree. C. for 1 hour, and
the concentration of the silica particle in the reaction system was
calculated from the weight after drying.

<Production of Abrasive (SA)>

[0086] An abrasive (SA) containing 3.0% by mass of the silica particles
produced in Example 1, 175 ppm of hydroxyethyl cellulose (HEC), and 225
ppm of ammonia was prepared.

<<Stability Test of Abrasive (Slurry)>>

[0087] The stability of the abrasive (slurry) was evaluated by the
presence or absence of white turbidity in the abrasive (SA) prepared in
the above <Production of abrasive (SA)>. The results are shown in
Table 1.

[0088] White turbidity was absent: Good

[0089] White turbidity was present: Poor

<<Polishing Test>>

[0090] The substrate for polishing (a single crystal silicon wafer having
a crystal structure of 1.0.0) was set in a polishing machine (NF300,
manufactured by Nano Factor Co., Ltd.), and polished for 10 minutes by
use of a polishing pad SUBA600 with a load of 15 kPa applied to the
substrate at a table rotation speed of 50 rpm, and a spindle speed of 60
rpm while the abrasive (SA) was fed at a rate of 250 mL/min. Thereafter,
the substrate was washed with pure water and air-dried.

[0091] Then, the polished surface of the resulting polished substrate was
observed, and the smoothness of the surface was evaluated by using the
following criteria (degree of scratches). The results are shown in Table
1.

[0092] Almost no scratch was found: Good

[0093] A few scratches were found: Fair

[0094] Scratches were found over a wide area: Poor

[0095] For silica components remaining on the polished substrate, the
degree of remaining was observed by using a laser microscope (VK-X250,
manufactured by KEYENCE CORPORATION), and evaluated by using the
following criteria. The results are shown in Table 1.

[0096] Almost no silica component remained: Good

[0097] A few silica components remained: Fair

[0098] Silica components remained over a wide area: Poor

Example 2

<Production of Dispersion Liquid of Silica Particle (SB) and Production
of Abrasive (SB)>

[0099] 310.0 g of methanol (liquid I) was kept at 50.degree. C., and
5703.8 g of a methanol solution of tetramethoxysilane (liquid A) and
1560.0 g of aqueous ammonia (liquid B) were simultaneously added to the
liquid I over 30 hours. After completion of the addition, the mixture was
further aged at the same temperature for 3 hours. The solvent was
substituted with pure water to obtain 20% by mass of a dispersion liquid
of a silica particle (SB). Detailed processing conditions and results of
various measurements are shown in Table 1. The chronological changes in
the variation rate of catalyst proportion and the variation rate of water
proportion are shown in FIG. 4.

[0100] An abrasive (SB) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (SB) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

Example 3

<Production of Dispersion Liquid of Silica Particle (SC) and Production
of Abrasive (SC)>

[0101] 410.0 g of methanol (liquid I) was kept at 40.degree. C., and 976.3
g of tetramethoxysilane (liquid A) and 1769.5 g of aqueous ammonia
(liquid B) were simultaneously added to the liquid I over 10 hours. After
completion of the addition, the mixture was further aged at the same
temperature for 3 hours. The solvent was substituted with pure water to
obtain 20% by mass of a dispersion liquid of a silica particle (SC).
Detailed processing conditions and results of various measurements are
shown in Table 1. The chronological changes in the variation rate of
catalyst proportion and the variation rate of water proportion are shown
in FIG. 5.

[0102] An abrasive (SC) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (SC) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

Example 4

<Production of Dispersion Liquid of Silica Particle (SD) and Production
of Abrasive (SD)>

[0103] 410.0 g of methanol (liquid I) was kept at 40.degree. C., and
2206.3 g of a methanol solution of tetramethoxysilane (liquid A) and
565.33 g of aqueous ammonia (liquid B) were simultaneously added to the
liquid I over 10 hours. After completion of the addition, the mixture was
further aged at the same temperature for 3 hours. The solvent was
substituted with pure water to obtain 20% by mass of a dispersion liquid
of a silica particle (SD). Detailed processing conditions and results of
various measurements are shown in Table 1. The chronological changes in
the variation rate of catalyst proportion and the variation rate of water
proportion are shown in FIG. 6.

[0104] An abrasive (SD) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (SD) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

Example 5

<Production of Dispersion Liquid of Silica Particle (SE) and Production
of Abrasive (SE)>

[0105] 410.0 g of methanol (liquid I) was kept at 50.degree. C., and 388.1
g of a methanol solution of tetramethoxysilane (liquid A) and 116.1 g of
aqueous ammonia (liquid B) were simultaneously added to the liquid I over
1 hour. After completion of the addition, the mixture was further aged at
the same temperature for 3 hours. The solvent was substituted with pure
water to obtain 20% by mass of a dispersion liquid of a silica particle
(SE). Detailed processing conditions and results of various measurements
are shown in Table 1. The chronological changes in the variation rate of
catalyst proportion and the variation rate of water proportion are shown
in FIG. 7.

[0106] An abrasive (SE) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (SE) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

Example 6

<Production of Dispersion Liquid of Silica Particle (SF) and Production
of Abrasive (SF)>

[0107] 410.0 g of methanol (liquid I) was kept at 50.degree. C., and
2328.6 g of a methanol solution of tetramethoxysilane (liquid A) and
696.69 g of aqueous ammonia (liquid B) were simultaneously added to the
liquid I over 6 hours. After completion of the addition, the mixture was
further aged at the same temperature for 3 hours. The solvent was
substituted with pure water to obtain 20% by mass of a dispersion liquid
of a silica particle (SF). Detailed processing conditions and results of
various measurements are shown in Table 1. The chronological changes in
the variation rate of catalyst proportion and the variation rate of water
proportion are shown in FIG. 8.

[0108] An abrasive (SF) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (SF) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

Comparative Example 1

<Production of Dispersion Liquid of Silica Particle (RA) and Production
of Abrasive (RA)>

[0109] A liquid I consisting of 2268.0 g of methanol, 337.5 g of pure
water, and 94.5 g of 29% aqueous ammonia was kept at 40.degree. C., and
2170.0 g of a methanol solution of tetramethoxysilane (liquid A) was
added to the liquid I over 160 minutes. After completion of the addition,
the mixture was further aged at the same temperature for 1 hour. The
solvent was substituted with pure water to obtain 20% by mass of a
dispersion liquid of a silica particle (RA). Detailed processing
conditions and results of various measurements are shown in Table 1. The
chronological changes in the variation rate of catalyst proportion and
the variation rate of water proportion are shown in FIG. 9.

[0110] An abrasive (RA) was produced in the same manner as in Example 1
except that the dispersion liquid of a silica particle (RA) was used, and
a stability test and a polishing test were performed in the same manner
as in Example 1. The results are shown in Table 1.

[0111] In any of the Examples and the Comparative Example, the silica
particle included a content of each of U and Th of less than 0.3 ppb, a
content of each of alkali metals, alkali earth metals, Fe, Ti, Zn, Pd,
Ag, Mn, Co, Mo, Sn, Al, and Zr of less than 0.1 ppm, and a content of
each of Cu, Ni, and Cr of less than 1 ppb.

<<Measurement of Contents of Metal Elements>>

[0112] The silica particle was dissolved in hydrofluoric acid, and heated
to remove the hydrofluoric acid, and then added with pure water as
necessary, and the resulting solution was subjected to measurement with
an inductively coupled plasma (ICP) emission mass spectrometer (e.g.,
ICPM-8500, manufactured by Shimadzu Corporation) to determine the
contents of metal elements in the silica particle.

[0113] As shown in Table 1, the dispersion liquid of a silica particle
produced in each of Examples 1 to 6 had less incompletely reacted
materials than that produced in Comparative Example 1 and also had
excellent slurry stability and polishing property.